The invention is a device and method for determining the current status and remaining life of a power source in an implantable neurological tissue stimulator. The invention contemplates a method, performed in a system without human intervention, that includes the steps of assessing the power source voltage of the power source in an IPG, determining, using the power source voltage, where in the power source life cycle the power source is, and taking action in response to the determination of where in the power source life cycle the power source is. The invention also includes a device that embodies the method described above. The device, which is partially resident in an IPG and partially resident in an external device such as a programmer measures the power source voltage in the IPG, using the power source voltage determines where in the power source life cycle the power source is and takes appropriate action in response to the determination of where in the power source life cycle the power source is. A processor, either on the IPG or in the programmer is the preferred structure for determining where in the power source life cycle the power source is and for directing the appropriate action in response to the determination of where in the power source life cycle the power source is.